1. Field of the Invention
The present invention relates to a transmission method, a transmission apparatus, a reception method and a reception apparatus suitable for application to a terminal apparatus of a radio telephone system, for example.
2. Description of the Related Art
In a mobile communication system such as a radio telephone system or the like, a multiple access in which a plurality of mobile stations (terminal apparatus or subscribers) are permitted to access a single base station is employed. In this case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, a code division multiple access (CDMA) system and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical carrier wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a base station receives it with taking code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spectrum, and transmits signals to a plurality of mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station discriminates each of the mobile stations by different spread codes one another.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in secrecy of telephone conversation. Therefore, the system is suitable for a radio transmission system utilizing mobile stations such as a portable telephone apparatus and so on.
In the CDMA system, it is difficult to establish a precise communication relationship between mobile stations. Therefore, each communication between respective mobile stations cannot be dealt completely separately, and hence another mobile station can become a source of interference upon communication with a mobile station. Further, data is spread within a particular frequency band in this system. Therefore, it is necessary to define a band width in advance over which the data is spread (i.e., a band width for use of transmission). Therefore, it is difficult to change the transmission band width.
The assignee of the present application has proposed a communication system called a band division multiple access (BDMA) system (disclosed in Japanese patent application No. 132434/1996 and so on) as another communication system. While details of the BDMA system will be described in the detailed description of the preferred embodiment later on, the BDMA system is as follows in short. A plurality of transmission bands in each of which subcarrier signals of a predetermined number are disposed at a predetermined frequency interval are prepared. A signal in each of the transmission bands is divided by every predetermined time to form time slots. A burst signal is transmitted in the form of a multicarrier signal modulated by dispersing the signal intermittently into the subcarrier signals of the above predetermined number at a period of the time slots of a predetermined number. This BDMA system has a very excellent transmission characteristic.
In a so-called cellular system in which a base station is allocated in each of cells, it is necessary for each of mobile stations to monitor the surrounding cells and to carry out a hand-off processing for switching a base station to a base station in an adjacent cell if necessary. However, when the surrounding cells are monitored, it is necessary to receive a control channel from a base station in an adjacent cell and to determine a reception level, a reception timing and so on based on the control channel. Therefore, it is necessary for a mobile station to temporarily stop the communication with a base station which it is communicating with and to then receive the control channel and so on from another base station. However, if the time of interruption of the communication with the base station with which it is communicating is set long, then the period of time during which it can communicate with the base station is lowered, which consequently lowers the transmission efficiency.